Preparation of isophthalic acid



I Patented Nov. 21, 19 50 PREPARATION OF ISOPHTHALIC ACID William G. Toland, Jr., Richmond, Calii., as-

signor to California Research Corporation, San Francisco, Calif., a corporation of Delaware No Drawing. Application November 17, 1947, Serial No. 786,549

4 Claims.

This invention relates to a method of preparing aromatic dicarboxylic acids, and particularly to a method of oxidizing an alkyl benzoic acid to produce an isophthalic acid.

The aromatic dibasic acids other than phthalic acid have, in the past, been limited to their industrial application by virtue of the fact that their known methods of preparation were only suited to small-scale laboratory equipment. The preparation of these acids, and particularly isophthalic acid, has been accomplished by chemical reagent methods, which require costly reagent materials and invariably result in low yields of desired product, along with difllcultly extractable intermediates and by-products.

It has now been discovered than an isophthalic acid may be prepared in economic yields by a non-catalytic method of preparation which involves a minimum of reaction variables and is well-suited to commercial production. According to the process of the invention, an isophthalic acid is produced by the oxidation of a benzene monocarboxylic acid possessing an alkyl group meta to the carboxyl group in the liquid phase with an oxygen-containing gas at temperatures of at least about 300 F. The dibasic acids, or isophthalic acids, thus produced possess two carboxyl groups metato one another and may contain additional substituents in the remaining positions in the benzene nucleus corresponding to the original substituents in the monocarboxylic acid charge stock.

The charge stock to the oxidation process may be any benzene monocarboxylic acid possessing an alkyl group meta to the carboxyl group. This alkyl group is not restricted in chain length, but, from the practical standpoint, the lower alkyl, groups, such as methyl, ethyl, and isopropyl, are to be desired. These meta alkyl benzoic acids may also contain additional substituents in one or more of the remaining positions of the nucleus, provided such substituents remain substantially inert and do not materially afiect the direction of the oxidation in the meta alkyl group. This description of the charge Stock is predicated upon the charging of a single compound, but it is to be understood that mixtures or compounds falling within the general class of charge compounds, or technical mixtures such as the isomers, in which the meta alkyl benzoic acids predominate, may be used in the reaction. In addition, partial oxidation products of the meta alkyl benzoic acids, such as m-acetyl benzoic acid, etc, are suitable charge compounds for the process.

In the preferred form of the invention, m-toluic acid and mixtures of isomeric toluic acids, in which m-toluic acid predominates, are used as the oxidation charge. Accordingly, m-toluic acid will be used as illustrative of a representative charge stock in the subsequent description of the subject process. Any modifications of the specific reaction conditions required when using charge stocks other than m-toluic acid will be obvious to those skilled in the art, in view of the present teachings.

It has been found that the oxidation of the meta alkyl benzoic acids to the desired dibasic proceeds at temperatures above 300 F. The optimum oxidation temperatures vary with the composition of the charge stock and generally lie within the range of from 330 to 550 F. Selection of the desired operating temperatures is dependent upon a balance of economic factors. Higher purity acids are produced at the lower operating temperatures, whereas the oxidation rate is increased at the higher temperatures. Additional factors which bear on the selection of the operating temperatures will become apparent from the further discussion of the operation of the process. In the oxidation of m-toluic acid to isophthalic acid, the optimum temperature range lies between 360 and 500 F., with the preferable range at atmospheric pressure between 420 and 440 F.

One of the important factors affecting the efficiency of the oxidation reaction i the solubility of the oxygen of the oxidizing gas in the liquid charge under the reaction conditions. For optimum conversion, a high solubility of oxygen is required. The greater the solubility, the faster the rate of reaction and the higher the yields of the desired dibasic acids. For a given charge compound at constant pressure, the degree of solubility is a function of the reaction temperature. Thus, as the temperature is increased, the oxygen 'solubility'is decreased. However, on application of pressure, such as 2 to 10 atmospheres, the oxygen solubility is increased. Accordingly, for effective oxidation in the upper range of the operating temperatures, it may be desirable to conduct the reaction under pressure. An additional advantage of a pressure reaction is the fact that less volatilization of charge compounds is obtained, and consequently, oxidizing gases containing inert constituents, such as air, may be used satisfactorily without incurring losses of charge stock due to sublimation and carry-over.

The subject process is applicable to either batch or continuous operation, and the equipment used may be similar to the conventional liquid phase oxidation process. The oxidizing gas should be introduced into the liquid charge in a finelydlspersed condition, such as is obtained through a turbo-mixer, iritted glass injector, etc., so as to approach complete physical saturation. The rate at which the oxidizing gas is introduced into the reactor should be controlled to correspond with the rate of solution or utilization of the oxygen, in order to provide a short contact time and minimize overoxidation and thermal decomposition.

The degree of conversion in the oxidation reaction is dependent upon the type of equipment and operation used, and the nature of the resulting products obtained. In the batch oxidation of m-toluic acid to isophthalic acid, the reaction is preferably controlled to no more than about a 60% conversion, since isophthalic acid is comparatively insoluble in the reaction medium, and,

at high concentrations, forms a thick slurry which is diflicult to handle. Furthermore, operating to high conversions results in decreased yields due to decarboxylation and overoxidation of the isophthalic acid. In the preferable batch operations, the reaction is conducted to a conversion of from -40%. Operations to higher overall conversions may be carried out in a continuous or semi-continuous operation where the isophthalic acids formed are continuously removed from the reaction, and unconverted charge stock is recycled to the reaction zone.

The separation and extractions of the desired dibasic acids may be carried out in an of the conventional and obvious manners. Due to the insolubility of isophthalic acid in m-toluic acid, the separation may be easily accomplished by filtration. Representative data, indicate that filtration of the oxidation mixture of isophthalic acid in m-toluic acid at temperatures of about 250-300 F. removes all but negligible amounts of isophthalic acid from the toluic acid filtrate. However, small amounts of toluic acid are retained by adsorption in the isophthalic acid filter cake and may be extracted by solvents such as xylenes, acetophenone, ethyl alcohol, etc. Color bodies which are almost universally present in the isophthalic acid product may, on the other hand, be removed by sublimation of the isophthalic acid or other means.

The following examples are presented as an illustration of the practice of the process of the invention.

Example I 272 grams (2 mols) of distilled m-toluic acid were introduced into a glass reactor 32 inches long and one inch in diameter. The charge stock was electrically heated to a temperature of 422 F. and maintained at that tcmperature with a tolerance of -7 F. throughout the reaction. A mixture of oxygen and nitrogen in the ratio of 4:1 was introduced into the bottom of the reactor through a capillary and bubbled through the molten acid for six hours at the rate of 0.3 cu. ft. per hour. At the end of the reac tion period, the reaction mixture was allowed to settle and the insoluble isophthalic acid filtercd oil. The filter cake was then refluxed with 400 cc. of xylene for 15 minutes, and again filtered and washed. The analysis of the reaction products, determined by the acid number, indicated a yield of 48 grams of isophthalic acid with 1.41 mols of m-toluic acid remaining unreacted.

Additional six-hour runs were made in accordance with the procedure or the foregoing example at varying temperatures of reaction. When holding the reaction temperature at 467 -5 F., a yield of 40 grams of isophthalic acid was obtained with 1.65 mols of toluic acid remaining unreacted. At 440 i10 F.. a yield of 42 grams formed with 1.61 mols of toluic acid remaining unreacted.

Example II 1224 grams (9 mols) of distilled m-toluic acid were heated in a glass reactor 72 inches long and 11% inches inside diameter. A mixture of oxygen and nitrogen in the ratio of 4:1 was introduced at the bottom of the reactor through a capillary and bubbled through the molten acid for a period of 24 hours at a rate of 0.3 cu. ft. per hour. The reactor was electrically heated and a reaction temperature of 415- :11 F. was maintained throughout the run. At the end of the 24-hour period, the reaction mass was drained through the bottom of the reactor and filtered at about 300 F. through a heated stainless steel funnel. The filter cake was then refiuxed for 20 minutes with 500 cc. of xylene. The xylene extract was filtered oil? and the xylene removed by distillation. Residual amounts of isophthalic and toluic acid were recovered by washing the equipment with acetone and distilling ofi the acetone. The conversion to isophthaiic acid was determined by the acid number and resulted in a yield of 204 grams of isophthalic acid with 7.50 mols of m-toluic acid remaining unreacted. The isophthalic acid isolated after xylene extraction had an acid number of 661, which corresponds to a purity of 94.8%.

Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and only such limitations should be imposed as are indicated in the appended claims.

Iclairn:

1. A process for the production of a. benzene meta dicarboxylic acid which comprises noncatalytically oxidizing a benzene monocarboxylic acid possessing an alkyl group meta to the carboxyl group in the liquid phase with an oxygencontaining gas at temperatures within the range of about 300 to 550 F.

2. A process for the production of a benzene meta dicarboxylic acid which comprises noncatalytically oxidizing a meta alkyl benzoic acid in the liquid phase with an oxygen-containing gas at temperatures within the range of about 300 to 550 F.

3. A process for the production of a benzene meta dicarboxylic acid which comprises noncatalytically oxidizing a meta methyl benzoic acid in the liquid phase with an oxygen-containing gas at temperatures within the range of about 300 to 550 F.

4. A process for the production of isophthalic acid which comprises noncatalytically oxidizing meta toluic acid in the liquid phase with an oxygen-containing gas at temperatures within the range of about 360 to 500 F.

WILLIAM G. TOLAND, JR.

(References on following page) 8 mmnncns cum The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date Pansegrau July 28, 1931 Jaeger Dec. 5, 1923 Mares June 14, 1938 Bone et al. May 7, 1940 m 8 Number Name Date 2,245,528 Loder June 10, 1941 2,276,774 Henke Mar. 17. 1942 OTHER REFERENCES- Weith et a1. Berlchte Deut. Chem, v01. 8, pa e "120 (1875) Graebe et 1., Berlchte Deut. Chem., vol. 39.

Whitmore: Organic Chemistry. page 828, D. Van Noatrand (70., Inc., N. Y., 1942. 

1. A PROCESS FOR THE PRODUCTION OF A BENZENE META DICARBOXYLIC ACIS WHICH COMPRISES NONCATALYTICALLY OXIDIZING A BENZENE MONOCARBOXYLIC ACID POSSESSING AN ALKYL GROUP META TO THE CARBOXYL GROUP IN THE LIQUID PHASE WITH AN OXYGENCONTAINING GAS AT TEMPERATURES WITHIN THE RANGE OF ABOUT 300 TO 550*F. 